The production of carbon fibers from acrylic fibers is generally conducted by a process which comprises heat-treating the latter fiber in an oxidizing atmosphere at 200.degree. to 400.degree. C. to form a flame-resistant structure and then carbonizing the resulting fiber in an inert atmosphere at a temperature not lower than 400.degree. C. In the above process, application of tension or elongation during the flame-resisting treatment is effective for producing carbon fibers having excellent tenacity and modulus of elasticity. For example, Japanese Patent Application Kokai (Laid-Open) No. 54,632/74 discloses a process to produce a high-performance carbon fiber by dividing the elongation during the flame-resisting treatment properly into that at the initial stage and that at the latter stage of the treatment.
However, acrylic fibers may sometimes give, depending on their initial molecular orientations or molecular cohesive forces, carbon fibers of more excellent performance when applied a shrinkage in the flame-resisting treatment rather than when applied an elongation. In the above-mentioned process, accordingly, excessive elongation may promote the development of fluff or structural defects. Thus, the optimum percentage of elongation or shrinkage in the flame-resisting treatment varies depending on the kind of precursors and is also influenced by the temperature of the atmosphere. Accordingly, it has been very difficult up to now to optimize the above conditions.
There have also been known a large number of proposals regarding the carbonization step.
For example, there is known a process disclosed in Japanese Patent Application Kokai (Laid-Open) No. 147,222/79. The process comprises subjecting a fiber which has been made flame-resistant and imparted a fiber density of 1.30 to 1.42 g/cm.sup.3 to a carbonization treatment in an inert atmosphere at a temperature region of 300.degree. to 800.degree. C. while applying an elongation in the range of 25% or less, and subsequently to a heat treatment at a temperature not lower than 800.degree. C. to obtain a carbon fiber. It is known that when a fiber which has been made flame-resistant is heat-treated under a constant load at a temperature not lower than 300.degree. C., the fiber undergoes a change of fiber length as shown in FIG. 1 in correspondence to the change of its density. In a heat-treatment region wherein the fiber density reaches about 1.50, the fiber itself undergoes a marked physical change and the structure of the fiber undergoes a complicated change. In conventional processes for producing carbon fibers, accordingly, the heat treatment has been conducted under such tension as to cause shrinkage of fiber length in order to prevent the occurring of troubles such as fiber breakage in said heat-treatment region. Such methods have been unable to produce carbon fibers of a high tenacity as described in the above-mentioned patent application, whereas the aforesaid invention has attained the object by the application of an elongation of up to 25% in said region. In said process, however, when a total elongation of up to 25% is applied, an extreme change of fiber length takes place, making uniform elongation treatment impossible. Therefore, it is very difficult to produce by the process carbon fibers showing uniform and high performance constantly.
As to the temperature-increase gradient, there is known a process disclosed, for example, in Japanese Patent Application Kokai (Laid-Open) No. 214,529/83. The process comprises subjecting a polyacrylonitrile-type fiber which has been made flame-resistant to heat treatment first in an inert atmosphere at 300.degree. to 700.degree. C. at a temperature-increasing rate of 100.degree. to 1100.degree. C./minute, then in an inert atmosphere through a region of 700.degree. C. to 1000.degree. C. at a temperature-increasing rate of 300.degree. to 5,000.degree. C./minute, and further in an inert atmosphere through a region of 1000.degree. to 1200.degree. C. at a temperature-increasing rate of 100.degree. to 1800.degree. C./minute to form a carbon fiber. However, the process involves as yet some points to be improved to become a process which can produce a high-tenacity carbon fiber having a tenacity of 400 kg/mm.sup.2 or more, particularly 450 kg/mm.sup.2 or more, with a narrow variation of quality and a high carbonization yield while suppressing the development of fluff to the minimum.
The studies on production of high-performance carbon fibers have been pursued from various aspects. It has been revealed that the most important point is to prevent the phenomena of fusion-bonding and agglutination between fibers in the flame-resisting treatment of the precursor. It is said that carbon fiber tow containing fusion-bonded fibers is of extremely low practical value even when the carbon fiber shows a single fiber property of a tenacity of 400 kg/mm.sup.2 and an elongation of 1.5% or more.
As to the prevention of fusion bonding and agglutination of the treated fibers in the flame-resisting step, there is disclosed in Japanese Patent Application Kokoku (Post-Exam. Publn) No. 24,136/77 a process to use a silicone-type textile oil as the oil for the precursor. However, the aminosiloxane-type oil disclosed in the above Application is still unsatisfactory for preventing fusion-bonding. This is due to the fact that owing to the fusion-bonding promotion effect of impurities such as emulsifier components contained in the aminosiloxane-type oil and also to the sticking effect of aminosiloxane the precursor is excessively collected, which results in insufficient fiber-separation of the precursor.